The present invention generally relates to spark plugs for spark ignition in an internal combustion engine. In particular, this invention is directed to a spark ignition assembly having a spark plug and ignition coil contained within a single housing, and a process for the manufacture of such an assembly.
With the exception of diesel engines, ignition systems for internal combustion engines typically involve igniting an air/fuel mixture within a combustion chamber with an electric spark jumped between electrodes of a spark plug. A single spark plug is typically threaded into a spark plug well for each cylinder of the engine. The high voltage required for spark ignition is typically stepped up from battery voltage with an ignition coil mounted separately within the engine compartment. Typically a single ignition coil services all of the spark plugs of an engine through a distributor.
In comparison to breaker-point systems, electronic ignition systems have significantly improved engine reliability, performance and efficiency while reducing maintenance through the use of a computer that coordinates fuel injection and ignition. In some more advanced electronic ignition systems, each spark plug is individually connected with a plug wire to an ignition coil paired exclusively with the plug, thus eliminating the need for a distributor. The computer individually triggers the coils to fire their respective plugs, using engine sensors to time the pulses correctly.
While ignition systems of the types described above are widely employed in the automobile industry, further improvements in automotive ignition systems are desirable, particularly if improved reliability can be achieved.
The present invention provides a spark ignition assembly for use in a spark ignition system of an internal combustion engine, and a method for manufacturing the spark ignition assembly. As with prior art spark plugs, the spark ignition assembly of this invention provides for the ignition of an air/fuel mixture within a combustion chamber by jumping a spark across a pair of electrodes. However, the spark ignition assembly of this invention improves system reliability by reducing the number of separate ignition system components that must be interconnected by external wiring.
The spark ignition assembly of this invention generally includes a housing within which a spark plug, ignition coil and connector are housed. The spark plug has an insulator body, a first electrode and a ground electrode at a first end of the insulator body, and a terminal at an oppositely-disposed second end of the insulator body. The ignition coil is electrically connected to the terminal of the spark plug and to the connector, through which an electric voltage is applied to the ignition coil. In a preferred embodiment, the housing contains a liquid injection molding (LIM) compound that surrounds the ignition coil and at least portions of the connector and spark plug. More particularly, the LIM compound preferably defines a boot that surrounds and grips the second end of the insulator body. Also in the preferred embodiment, the ignition coil employs an epoxy-impregnated secondary winding in order to ensure a void-free winding with adhesion to the secondary spool.
In the method of the present invention, the spark ignition assembly is manufactured by assembling the connector, the ignition coil and the spark plug in the housing so that the connector is electrically connected to the ignition coil for applying battery voltage to the coil, and the coil is electrically connected to the terminal of the spark plug for applying a stepped-up voltage to the plug. According to the preferred embodiment, the ignition coil and at least a portion of the connector is encased in the LIM compound prior to being assembled with the spark plug in the housing. Also in the preferred embodiment, the ignition coil and at least a portion of the connector is surrounded by a plastic case to form a subassembly, which then undergoes liquid injection molding prior to being assembled with the spark plug in the housing. During injection molding, a portion of the LIM compound is molded outside of the case to define the boot that grips the spark plug and therefore helps secure the plug to the housing.
In accordance with the above, the spark ignition assembly of this invention has the advantage of having a modular construction with fewer individual and separate components that must be interconnected with wires. In the preferred embodiment, the LIM compound encapsulates the ignition coil and a portion of the connector to form an environmentally protective encasement within the housing. The spark ignition assembly of this invention also has the advantage of being a reliable and robust product that can be assembled with relatively simple equipment and low capital investment. The spark plug assembly simplifies engine installation and provides a more cost-effective manufacturing process.
Other objects and advantages of this invention will be better appreciated from the following detailed description.